1. Field of the Invention
The present invention relates to a semiconductor apparatus. More particularly, the present invention is concerned with a semiconductor apparatus for fingerprint recognition used as a capacitive mode fingerprint sensor.
2. Description of the Related Art
In recent years, the fingerprint matching system, which has conventionally been utilized in the applications of the administration of entrance and exit and the like, is being attracting attention as a security system for the computer network and a personal identification tool in a portable terminal or the like. Examples of fingerprint detecting methods using the fingerprint matching system include an optical detection method, and a capacitive sensing method disclosed in Japanese Patent Application Laid-Open Specification No. 4-231803, whose basic application is GB9011163 which is also a basic application of U.S. Pat. No. 5,325,442.
The capacitive sensing method is one in which an electrostatic capacity value between the electrodes in the fingerprint sensor is detected, and has an advantage in that the apparatus employing the capacitive sensing method is easily down-sized, and therefore, this method is advantageously mounted on a portable terminal and the like. Thus, the development of the capacitive mode fingerprint sensor is energetically progressed.
FIG. 4 is a diagrammatic cross-sectional view of a semiconductor device for fingerprint recognition, which constitutes the above-mentioned capacitive mode fingerprint sensor.
On a substrate having formed thereon a semiconductor device constituting the sensor, such as a transistor or the like (see FIG. 5A described blow), a barrier metal comprised of titanium (Ti) or the like is formed. On the barrier metal, electrodes 52 comprised of, for example, aluminum or the like are formed so that they are arranged in a matrix form and connected to the above-mentioned semiconductor device. Further, pad electrodes 52a are formed simultaneously with conducting the step of forming the electrodes 52.
An insulating protecting film 53 is formed so as to cover both of the electrodes 52 and the pad electrodes 52a, and an opening portion is formed in each of the pad electrodes 52a. Thus, a fingerprint recognition semiconductor chip 51 using, as a fingerprint-recognizing surface, a region in which the electrodes 52 are arranged in a matrix form is formed.
The pad electrodes 52a formed in the fingerprint recognition semiconductor chip 51 and the lead 55 are connected to each other by a wire bonding 54.
While exposing the fingerprint-recognizing surface (upper surface) of the fingerprint recognition semiconductor chip 51 to the outside, the wire bonding 54 which connects the fingerprint recognition semiconductor chip 51 and the lead 55 is encapsulated by a mold resin 56 comprised of, for example, a thermosetting resin or the like.
Next, an explanation is made on the principle of the operation of the semiconductor device for fingerprint recognition.
FIG. 5A is an enlarged, cross-sectional view of the portion of the electrodes (corresponding to reference numeral 52 in FIG. 4) which are formed in the semiconductor chip (corresponding to reference numeral 51 in FIG. 4) of the semiconductor device for fingerprint recognition so that they are arranged in a matrix form.
On a substrate 10 having formed thereon a semiconductor device constituting the sensor, such as a transistor (not shown) or the like, a barrier metal 20 comprised of Ti or the like is formed. On the barrier metal 20, charge storage electrodes 21 comprised of, for example, aluminum or the like are formed so that they are arranged in a matrix form and connected to the above-mentioned (not shown) semiconductor device on the substrate. An insulating protecting film 30 is formed so as to cover the charge storage electrodes 21.
As shown in FIG. 5A, when a finger 7 is in contact with the fingerprint-recognizing surface of the semiconductor apparatus for fingerprint recognition, a capacitor is formed between the charge storage electrodes 21, the insulating protecting film 30 and the finger 7. The insulating protecting film 30 functions as a part of the capacitor insulating film. In the construction mentioned above, the distance d (for example, d1, d2) between the charge storage electrodes 21 and the finger 7 varies depending on fingerprint unevenness 70. Accordingly, a difference in capacity is caused between the capacitors which constitute the fingerprint sensor and which are formed to be arranged in a matrix form, and thus, the charge stored in each of the charge storage electrodes 21 is read and detected by the semiconductor device formed on the substrate 10, such as a transistor or the like, making it possible to recognize a fingerprint.
In the above semiconductor apparatus, each of the charge storage electrodes 21 constitutes a unit cell of the fingerprint-recognizing surface of the semiconductor apparatus for fingerprint recognition.
In a state such that the finger is not in contact with the fingerprint-recognizing surface, the capacitors constituted by the charge storage electrodes 21 and the like have a “d” value which is infinite (∞) in all of the unit cells of the fingerprint-recognizing surface of the semiconductor apparatus for fingerprint recognition. Therefore, in all of the unit cells, the electrostatic capacity value Cs is zero (0).
On the other hand, in a state such that the finger is in contact with the fingerprint-recognizing surface, as shown in FIG. 5B, in the n-th unit cell, a capacitor having an electrostatic capacity value Csn is formed between the charge storage electrodes 21, the insulating protecting film 30 and the finger 7. The electrostatic capacity value Csn is represented by the formula: Csn=ε·ε0·S/dn. In this formula, S is an area contributing to the capacitor of each electrode, dn is a distance (for example, d1, d2) between the electrode of the n-th unit cell and the finger, and n is the number (n=1, 2, . . . ) of the unit cell.
In the construction for reading the electrostatic capacity value Csn in each of the unit cells, the capacitor formed between the charge storage electrode 21 of each unit cell, the insulating protecting film 30 and the finger 7 is connected to one source-drain region of the transistor which is gate-controlled by, for example, a word line WL (WL1, WL2, . . . ), and another source-drain region is connected to a bit line BL (BL1, BL2, . . . ), and further, a capacitor having an electrostatic capacity value CB is connected to the bit line BL.
In the above construction, when a finger is in contact with the recognition surface in a state such that a potential Vcc is applied to the bit line BL (Vcc precharge), a potential change represented by the formula: ΔVn=[Csn/(CB+Csn)] Vcc is caused in the bit line BL. The potential change ΔVn is detected in each of the unit cells, and the electrostatic capacity value Csn per unit cell is calculated, so that the fingerprint recognition is conducted by image processing or the like.
However, the conventional semiconductor device for fingerprint recognition mentioned above poses a problem in that, when a finger is in contact with the fingerprint-recognizing surface, the static electricity charged in the human body is discharged in the electrodes 52 (21) and a large amount of a current flows through the electrodes 52 (21) into the detecting circuit formed on the same semiconductor substrate (see FIG. 5B), so that the circuit is damaged and the function as the semiconductor apparatus for fingerprint recognition is lost.
On the other hand, as mentioned above, the insulating protecting film 53 (30) for the surface of the semiconductor apparatus for fingerprint recognition functions as a part of the capacitor insulating film of the capacitor formed between the electrodes 52 (21), the insulating protecting film 53 (30) and the finger 7. Therefore, a range within which the thickness of the insulating protecting film 53 (30) is increased and the material for the insulating protecting film 53 (30) is changed for the purpose of suppressing the damage of the circuit due to the discharge of static electricity is inevitably limited.